Rotoscoping to a transparent background
There is no need to make the background white (or green) when you can make it transparent without more processing. The idea is to use the mask information to determine which parts of the image are opaque and which are transparent.  

Don't use the mix node. 

Use Set Alpha, so that your mask controls the transparency of the video: Whatever is white on the mask will remain opaque, the black areas will be transparent.



If you need to re-position or re-scale the elements do so after the set alpha node, so that the transformation happen as a single combined element.